Polycarbonates are commercially important materials possessing excellent physical and chemical properties which are useful in a wide range application from non-opaque impact resistant sheets to shaped articles. Generally, however, polycarbonates have rather low scratch resistance and are somewhat susceptible to attack by many common solvents and chemicals.
Previous efforts to overcome this low scratch resistance and susceptibility to attack by solvents have included lamination procedures and application onto the polycarbonate of a surface coating. Many of these prior art remedial efforts have been unsuccessful due to the incompatibility of the laminae and coating materials with the polycarbonate substrate. This incompatibility has resulted in stress cracking and crazing of the polycarbonate, crack propagation into the polycarbonate as a result of the brittleness of the coating, and a reduction of the advantageous properties of the polycarbonate such as, for example, impact resistance, tensile strength, non-opacity and elongation.
The prior art coatings for polycarbonates have included organopolysiloxaner, U.S. Pat. No. 3,707,397; polyester-melamines or acrylic-melamines, U.S. Pat. No. 3,843,390; and allyl resins, U.S. Pat. No. 2,332,461. These types of prior art coatings are generally applied from solutions of inert solvents and are cured to final properties by baking at elevated temperatures. The disadvantages of such systems are obvious. The heat curing requires a supply of thermal energy thereby adding to the cost of the system. Further, the thermal curing step is somewhat limited by the heat distortion temperature of the polycarbonate which is to be coated. Thus, in coating of polycarbonates, sheets of 30 mils and less generally cannot be coated and cured economically because of excessive warpage of the sheets during the thermal curing process.
It is also known in the art to impart abrasion resistance to the surface of a synthetic resin article by forming on the surface of said article a cured film containing, inter alia, a mixture of a compound having a total of at least three acryloxy and/or methacryloxy groups and a copolymerizable mono- or diethylenically unsaturated compound. Thus, for example, U.S. Pat. No. 3,968,305 describes a synthetic resin shaped article having a mar-resistant polymer surface layer consisting essentially of, in polymerized form, (a) 20 to 100 weight % of a compound having a total of at least three acryloxy and/or methacryloxy groups linked with a straight chain aliphatic hydrocarbon residue having not more than 20 carbon atoms and (b) 0 to 80 weight % of at least one copolymerizable mono- or diethylenically unsaturated compound. U.S. Pat. No. 3,968,309 describes a molded article of plastic having on its surface a cured film of a coating material comprising at least 30% by weight of at least one polyfunctional compound selected from the group consisting of polymethacryloxy compounds having at least three methacryloyloxy groups in one molecule and a molecular weight of 250 to 800 and polyacryloxy compounds having at least three acryloyloxy groups in one molecule and a molecular weight of 250 to 800, and a fluorine-containing surfactant, in which the fluorine atom is bonded to a carbon atom.
It has now been found that certain specific polyfunctional acrylic monomers in conjunction with certain acrylic modified polymers can be used to provide excellent UV cured coatings for polycarbonate. Thus, the present invention provides a certain acrylate ester monomer-acrylic modified polymer based UV-cured non-opaque coating for polycarbonates which has improved adhesion, especially after exposure to weathering, to the polycarbonate, is compatible with the polycarbonate, and is mar, abrasion and solvent resistant.